1. Field of the Invention
The present invention relates to data communication by a CATV communication system, and more particularly to a CATV communication system including a center device and terminal devices, and a method of performing data communication by means of the CATV communication system.
2. Description of the Prior Art
There are conceivable some methods of the data communication by using a duplex or bi-directional CATV communication system. One of them is the packet communication method with collision detection function, which is generally employed in a LAN or the like. Now, it is assumed that a user A sends data to another user B via the CATV wired line. According to this method, the user A sends data via the upstream transmission line to the center device, which returns data thus received to send it to all the terminal devices, including the terminal device of the user B, via the downstream transmission line. If no collision takes place, the user A, the sender, refers to data returned via the downstream transmission line and determines the coincidence of returned data with data that he has sent, thereby confirming the correct data transmission being executed. On the other hand, if plural users send data, respectively, almost simultaneously, data collision likely takes place. FIG. 7 schematically illustrates the manner of the data collision, wherein the user A sent data Da via the upstream transmission line, but the user B also sent data Db almost at the same time. Data Da and data Db collide with each other to be broken, and consequently data Dx different from original data Da and Db is generated. Data Dx is received by the center device, which returns it to the downstream transmission line. The users A and B detect discordance between data that they have sent, respectively, and data returned, and recognize the collision has taken place.
An alternative way for the data communication via the CATV communication system is to use the timeslot system which is generally illustrated in FIG. 8. This system divides the upstream transmission line and the downstream transmission line into a plurality of unit time (generally called "timeslot") each having a predetermined time width, and a timeslot is assigned to a user. For example, assuming now that the eighth timeslot (oblique portion in FIG. 8) is assigned to the user A, he uses the eighth timeslot thus assigned for the data communication thereafter. Large capacity data may be transmitted by repeatedly sending the divided portions of data at the successive cycles of the eighth timeslot each having a limited data capacity.
However, the packet communication method with collision detection function described above has the following drawbacks. First, the collision occurrence is not found until broken data is received in the downstream transmission line by the sender, and hence the collision detection generally delays. Second, the users send data of random data length, respectively, and hence, when a user sends a large data, other users have to wait for a long time until his data transmission ends. Third, since downstream data cannot be transmitted while the upstream data is returned by the center device for the purpose of the collision detection, a complete duplex communication cannot be achieved.
On the other hand, the timeslot communication system has the following disadvantages. First, since the limited number of timeslots are assigned to the users for the exclusive use, only a limited number of users can make the data transmission simultaneously. Second, generally, the user does not actually send or receive data for a continuous long time period. Even when two users are communicating with each other by sending and receiving data, data is not actually running through the transmission line for a long time period, and the users are not sending or receiving data but referring to the received data or preparing the response in the major time period. But even in such a period in which data is not actually running through the transmission line, the transmission line is still kept connected, and hence the transmission line is not very efficiently used.